Eusebius. The Ecclesiastical History, translated from Greek by Kirsopp Lake and J.E.L. Oulton (New York, 1926, 1932), in 1075 bookmarked and searchable pdf pages. Loeb Classical Library volumes L153 and L265 in one file. Wikipedia has a main entry on Eusebius of Caesarea Palestina (ca. 263-339), and a group of entries about his writings. Eusebius describes the emergence of the Christian church, its teachers, leaders, heresies, persecutions and eventual dominance under the Byzantine emperor Constantine. Eusebius was an eyewitness to some of the events described in his book and had access to archival documents for other sections. The Ecclesiastical History was the first full-length historical narrative written from a Christian point of view. Attached to the document is a multilingual HTML version made from Arthur Cushman McGiffert's earlier translation (1895). Prepared by Robert Bedrosian. Internet Archive has a selection of works by or about Eusebius.

Note: the Greek and English texts are best displayed with the pages "side by side" ("two up" or "facing"). Pdf readers (such as Foxit and Adobe) have this option under the menu item View/Page Display.